In SimBiology, a species is intended to represent a entity (like a protein or some other chemical) that exists in a physical compartment that has a capacity (for example, a volume). So species units must be either in amout or concentration. This is what allows SimBiology to do mass balances for all the species participating in reactions.
If you want to model a state that has some other units, then you can do so by making it a parameter. If the value needs to change over time, then just be sure to set the Constant property to false. You can change the value of such a parameter using rules and events.
If you still have questions, I suggest sharing more details about your model. What is this thing that has units of (molecules/time) and how is it used in your model?
